Method for preparing spent nuclear fuel for dry storage

ABSTRACT

A system and method for drying cavities containing spent nuclear fuel is devised. The invention utilizes a non-intrusive procedure that is based on monitoring the dew point temperature of a non-reactive gas that is circulated through the cavity. In one aspect, the invention is a system for drying a cavity loaded with spent nuclear fuel comprising: a canister forming the cavity, the cavity having an inlet and an outlet; a source of non-reactive gas; means for flowing the non-reactive gas from the source of non-reactive gas through the cavity; and means for repetitively measuring the dew point temperature of the non-reactive gas exiting the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed as a continuation of U.S. patent application Ser.No. 14/288,045 filed May 27, 2014, which is a continuation applicationto U.S. patent application Ser. No. 13/622,343, filed Sep. 18, 2012(issued as U.S. Pat. No. 8,737,559), which is a continuation applicationof U.S. patent application Ser. No. 12/641,378, filed Dec. 18, 2009(issued as U.S. Pat. No. 8,266,823), which is a divisional applicationof U.S. patent application Ser. No. 11/145,785, filed on Jun. 6, 2005(issued as U.S. Pat. No. 7,707,741). The aforementioned prioritydocuments are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to the field of storing spentnuclear fuel, and specifically to the field of preparing spent nuclearfuel for dry storage.

BACKGROUND OF THE INVENTION

The storage, handling, and transfer of HLW, such as spent nuclear fuel,requires special care and procedural safeguards. In the operation ofnuclear reactors, hollow zircaloy tubes filled with enriched uranium,known as fuel assemblies, are burned up inside the nuclear reactor core.It is customary to remove these fuel assemblies from the reactor aftertheir energy has been depleted down to a predetermined level. Upondepletion and subsequent removal, this spent nuclear fuel (“SNF”) isstill highly radioactive and produces considerable heat, requiring thatgreat care be taken in its subsequent packaging, transporting, andstoring. Specifically, the SNF emits extremely dangerous neutrons andgamma photons. It is imperative that these neutrons and gamma photons becontained at all times subsequent to removal from the reactor core.

In defueling a nuclear reactor, it is common place to remove the SNFfrom the reactor and place the SNF under water, in what is generallyknown as a spent fuel pool or pond store. The pool water facilitatescooling of the SNF and provides adequate radiation shielding. The SNF isstored in the pool for a period long enough to allow the decay of heatand radiation to a sufficiently low level to allow the SNF to betransported with safety. However, because of safety, space, and economicconcerns, use of the pool alone is not satisfactory when the SNF needsto be stored for a considerable length of time. Thus, when long-termstorage of SNF is required, it is standard practice in the nuclearindustry to store the SNF in a dry state subsequent to a brief storageperiod in the spent fuel pool, i.e., storing the SNF in a dry inert gasatmosphere encased within a structure that provides adequate radiationshielding. One typical structure that is used to store SNF for longperiods of time in the dry state is a storage cask.

Storage casks have a cavity suitably sized to receive a canister of SNFand are designed to be large, heavy structures made of steel, lead,concrete and an environmentally suitable hydrogenous material.Typically, storage casks weigh about 150 tons and have a height greaterthan 15 ft. A common problem associated with storage casks is that theyare too heavy to be lifted by most nuclear power plant cranes. Anothercommon problem is that storage casks are generally too large to beplaced in spent fuel pools. Thus, in order to store SNF in a storagecask subsequent to being cooled in the pool, the SNF must be removedfrom the pool, prepared in a staging area, and transported to thestorage cask. Adequate radiation shielding is needed throughout allstages of this transfer procedure.

As a result of the SNF's need for removal from the spent fuel pool andadditional transportation to a storage cask, an open canister istypically submerged in the spent fuel pool prior to the SNF beingremoved from the reactor core. The SNF is then placed directly into theopen canister while submerged in the water. However, even after sealing,the canister alone does not provide adequate containment of the SNF'sradiation. A loaded canister cannot be removed or transported from thespent fuel pool without additional radiation shielding. Thus, apparatusand methods that provide additional radiation shielding during thetransport of the SNF have been developed. The additional radiationshielding is typically achieved by positioning the canisters in largecylindrical containers called transfer casks while submerged within thepool. Similar to storage casks, transfer casks have a cavity suitablysized to receive the canister and are designed to shield the environmentfrom the radiation emitted by the SNF within.

In facilities utilizing transfer casks to transport loaded canisters, anempty canister is first placed into the cavity of an open transfer cask.The canister and transfer cask are then submerged in the spent fuelpool. Previously discharged SNF from reactors located in wet storage ismoved into the submerged canister (which is within the transfer cask andfilled with water). The loaded canister is then fitted with its lid,enclosing the SNF and the water from the pool within the canister. Theloaded canister and transfer cask are then removed from the pool by acrane and set down in a staging area to prepare the SNF-loaded canisterfor storage or transportation in a dry condition. In order for anSNF-loaded canister to be properly prepared for dry storage ortransportation, the United States Nuclear Regulatory Commission (“NRC”)requires that the SNF and interior of the canister be adequately driedbefore the canister is sealed and transferred to the storage cask.Specifically, NRC regulations mandate that the vapor pressure (“vP”)within the canister be at or below 3 Torr (1 Torr=1 mm Hg) before thecanister is backfilled with an inert gas and sealed. Vapor pressure isthe pressure of the vapor over a liquid at equilibrium, whereinequilibrium is defined as that condition where an equal number ofmolecules are transforming from the liquid phase to gas phase as thereare molecules transforming from the gas phase to liquid phase. Requiringa low vP of 3 Torr or less assures an adequately dry space in thecanister interior suitable for long-term SNF storage or transportation.

Currently, nuclear facilities comply with the NRC's 3 Torr or less vPrequirement by performing a vacuum drying process. In performing thisprocess, the bulk water that is within the canister is first drainedfrom the canister. Once the bulk of the liquid water is drained, avacuum system is coupled to the canister and activated so as to create asub-atmospheric pressure condition within the canister. Thesub-atmospheric condition within the canister facilitates evaporation ofthe remaining liquid water while the vacuum helps remove the watervapor. The vP within the canister is empirically ascertained through avacuum-and-hold procedure. If necessary, the vacuum-and-hold procedureis repeated until the pressure rise during a prescribed test duration(30 minutes) is limited to 3 Torr. Once the vacuum drying passes theacceptance test, the canister is backfilled with an inert gas and thecanister is sealed. The transfer cask (with the canister therein) isthen transported to a position above a storage cask and the SNF-loadedcanister is transferred into the storage for long-term storage.

Current methods of satisfying the NRC's 3 Torr or less vP requirementare time consuming, manually intensive and prone to error from line andvalve leakages. Any time the canister must be physically approached forvacuum monitoring and dryness testing, there is the risk of exposing thework personnel to high radiation. Moreover, the creation ofsub-atmospheric conditions in the canister requires expensive vacuumequipment and can cause complicated equipment problems.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand system for drying a canister loaded with HLW.

Another object of the present invention is to provide a method andsystem for drying a canister loaded with HLW without physicallyaccessing the contents of the canister to ensure that an acceptablelevel of dryness has reached within the canister.

Yet another object of the present invention is to provide a method andsystem for drying a canister loaded with HLW without subjecting theinterior of the canister to sub-atmospheric conditions.

Still another object of the present invention is to provide a method andsystem for drying a canister loaded with HLW without using expensivevacuum equipment.

A further object of the present invention is to provide a method andsystem for preparing an SNF-loaded canister for dry storage that is easyto implement and/or time efficient.

A yet further object of the present invention is to provide a method andsystem for preparing a canister loaded with HLW for dry storage in amore cost effective manner.

These objects and other objects are met by the present invention whichin one aspect is a method of drying a cavity loaded with “HLW”comprising: a) flowing a non-reactive gas through the cavity; b)repetitively measuring dew point temperature of the nonreactive gasexiting the cavity; and c) upon the dew point temperature of thenon-reactive gas exiting the cavity being measured to be at or below apredetermined dew point temperature for a predetermined time,discontinuing the flow of the non-reactive gas and sealing the cavity.

By ensuring that the non-reactive gas coming out of the cavity has a dewpoint temperature that is at or below the predetermined dew pointtemperature for the predetermined period of time, it is ensured that thecavity is adequately dry (i.e., that the vP of the non-reactive gaswithin the cavity is below a desired level without the need tophysically measure the vP therein).

In some embodiments, the predetermined dew-point temperature is selectedso that a desired vapor pressure is achieved within the cavity, such as3 Torr or less.

The flow rate of the non-reactive gas through the cavity determines thepredetermined time for a specified dryness level (i.e., a predetermineddew point temperature). The predetermined dew point temperature and thepredetermined time for any sized cavity volume canister can bedetermined through experimentation or simulation.

In some embodiments, the inventive method may further comprise the stepsof: d) drying the non-reactive gas that exits the cavity after the dewpoint temperature is measured; and e) re-circulating the driednon-reactive gas through the cavity. The drying step can be performed bycontacting the non-reactive gas with a desiccant or by chilling thenon-reactive gas.

In some embodiments, the non-reactive gas will be circulated through thecavity at a predetermined flow rate. The predetermined flow rate can bechosen so that the volume of the cavity is turned over 25 to 50 timesduring the predetermined time.

In some embodiments, the predetermined dew point temperature can be in arange of approximately 20 to 26° F., and the predetermined time is in arange of approximately 25 to 35 minutes. In one embodiment, it ispreferred that the predetermined dew point temperature be approximately22.9° F. and the predetermined time be approximately 30 minutes.

Suitable non-reactive gases include, without limitation, nitrogen,carbon dioxide, light hydrocarbon gases, or a noble gas selected from agroup consisting of helium, argon, neon, radon, krypton, and xenon.

In another aspect, the invention can be a system for drying a cavityloaded with HLW comprising a canister forming the cavity, the cavityhaving an inlet and an outlet; a source of non-reactive gas; means forflowing the non-reactive gas from the source of non-reactive gas throughthe cavity; and means for repetitively measuring the dew pointtemperature of the non-reactive gas exiting the cavity. The dew pointtemperature measuring means can be any type of a direct moisture-sensingdevice, e.g., a hygrometer, or by other means, e.g., gas chromatography,mass spectroscopy etc.

In some embodiments, the system can further comprise means for dryingthe nonreactive gas. Suitable drying means include the use of a chiller,freezer, and/or condenser or the use of desiccant. In such anembodiment, the drying means will be located downstream of the dew pointtemperature measuring means. Embodiments of the system that comprises adrying means can also comprise means for re-circulating the desirednon-reactive gas from the drying means back into the non-reactive gassource. This can be accomplished through the use of a recirculationline.

In some embodiments, the system can be automated, and will furtherinclude: a controller operably coupled to the dew point temperaturemeasuring means. In such an embodiment, the dew point temperaturemeasuring means is preferably adapted to create signals indicative ofthe measured dew point temperature of the non-reactive gas and transmitthe signals to the controller. The controller is adapted to analyze thesignals and upon determining that the signals indicate that the measureddew point temperature is at or below the predetermined dew pointtemperature for the predetermined time, the controller is furtheradapted to (1) cease flow of the non-reactive gas through the cavity;and/or (2) activate a means for indicating that the cavity is dry.

In one embodiment, the system will further comprise a spent fuel cask.In such an embodiment, the canister will be positioned and dried withinthe cask.

Finally, it is preferred that the cavity have a top and a bottom, andthat an inlet be located at or near the bottom of the cavity forsupplying the non-reactive gas to the cavity and that an outlet forremoving the wet non-reactive gas from the cavity be located at or nearthe top of the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of an open canister thatcan be used in conjunction with the present invention shown partially insection and empty.

FIG. 2 is a perspective view of a transfer cask partially in sectionwith the canister of FIG. 1 sealed and positioned in the transfer cask.

FIG. 3 is a schematic diagram of a closed-loop system according to thepresent invention.

FIG. 4 is a flowchart of a first embodiment of a method of drying acanister loaded with SNF according to the present invention and usingthe system of FIG. 3.

FIG. 5 is a chart plotting the relationship between dew pointtemperature and vapor pressure for helium gas that can be used todetermine a target dew point temperature according to one embodiment ofthe present invention.

FIG. 6 is a chart plotting the relationship between dew pointtemperature within a canister and time when subjected to a flow ofhelium gas according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a canister 20 that is suitable for use with thepresent invention, The present invention is not limited to specificcanister geometries, structures, or dimensions and is applicable to anytype of enclosure vessel used to transport, store, or hold radioactiveelements. While the exemplified embodiment of the invention will bedescribed in terms of its use to dry a canister of spent nuclear fuel(“SNF”), it will be appreciated by those skilled in the art that thesystems and methods described herein can be used to dry radioactivewaste in other forms and in a variety of different containmentstructures as desired.

The canister 20 comprises a bottom plate 22 and a cylindrical wall 24which forms a cavity 21. As used herein, the end 25 of the canister 20that is closest to the bottom plate 22 will be referred to as the bottomof the canister 20 while the end 26 of the canister 20 that is furthestfrom the bottom plate 22 will be referred to as the top of the canister20. The cavity 21 has a honeycomb grid 23 positioned therein. Thehoneycomb grid 23 comprises a plurality of rectangular boxes adapted toreceive spent nuclear fuel (“SNF”) rods. The invention is not limited bythe presence of the honeycomb grid.

The canister 20 further comprises a drain pipe with an open bottom (notillustrated) located at or near the bottom of the canister 20 thatprovides a sealable passageway from outside of the canister 20 to theinterior of the cavity 21. If desired, the drain opening can be locatedin the bottom plate 22 or near the bottom of the canister wall. Thedrain pipe can be opened or hermetically sealed using conventionalplugs, drain valves, or welding procedures.

As illustrated in FIG. 1, the canister 20 is empty (i.e. the cavity 21does not have SNF rods placed in the honeycomb grid 23) and the top 26of the canister 20 is open. In utilizing the canister 20 to transportand store SNF rods, the canister 20 is placed inside a transfer cask 10(FIG. 2) while the canister 20 is open and empty. The open transfer cask10, which is holding the open canister 20, is then submerged into aspent fuel pool which causes the volume of the cavity 21 to becomefilled with water. SNF rods that are removed from the nuclear reactorare then moved under water from the spent fuel pool and placed insidethe cavity 21 of the canister 20. Preferably, a single bundle of SNFrods is placed in each rectangular box of the honeycomb grid 23. Oncethe cavity 21 is fully loaded with the SNF rods, the canister lid 27(FIG. 2) is positioned atop the canister 20. The canister lid 27 has aplurality of sealable lid holes 28 that form a passageway into thecavity 21 from outside of the canister 20 when open. The transfer cask10 (having the loaded canister 20 therein) is then lifted from the spentfuel pool by a crane and placed uprightly in a staging area (as shown inFIG. 2) so that the canister 20 can be properly prepared forthy-storage. This dry-storage preparation includes drying the interiorof the canister 20 and sealing the lid 27 thereto.

Referring now to FIG. 2 exclusively, when in the staging area, thecanister 20 (containing the SNF rods and pool water) is within thetransfer cask 10. Both the canister 20 and the transfer cask 10 are inan upright position. Once in the staging area, the drain pipe attachedto the canister lid 27 (not illustrated) with a bottom opening at ornear the bottom 25 of the canister 20 is used to expel the bulk waterthat is trapped in the cavity 21 of the canister 20 using a blowdown gas(usually helium or nitrogen). Despite draining the bulk water from thecavity 21, residual moisture remains in the cavity 21 and on the SNFrods. However, before the canister 20 can be permanently sealed andtransported to a storage cask for long-term dry storage ortransportation, it must be assured that that cavity 21 and the SNF rodscontained therein are adequately dried. Because a low vapor pressure(“vP”) within a container indicates that a low level of moisture ispresent, the United States Nuclear Regulatory Commission (“NRC”)requires compliance to the 3 Torr or less vapor pressure (“vP”)specification within the cavity 21 of HLW containing casks.

FIG. 3 is a schematic of an embodiment of a closed-loop drying system300 capable of drying the cavity 21 to acceptable NRC levels without theneed to intrusively measure the resulting vP within the cavity 21. Oncethe transfer cask 10, which is holding the canister 20, is positioned inthe staging area and the bulk water is drained from the cavity 21, thedrying system 300 is connected to the inlet 28 and outlet 29 of thecanister 20 so as to form a closed-loop system. More specifically, thegas supply line 325 is fluidly connected to the inlet 28 of the canister20 while the gas exhaust line 326 is fluidly connected to the outlet 29of the canister 20. The inlet 28 and outlet 29 of the canister are mereholes in the canister 20. If desired, proper port connections, seals,and/or valves can be incorporated into the inlet and outlet 28, 29.

The drying system 300 comprises a non-reactive gas reservoir 310, asupply pump 320, a flow rate valve 321, a dew point temperaturehygrometer 330, a chiller 340, a recirculation pump 360, and a controlsystem 350, which includes a suitably programmed microprocessor 351, acomputer memory medium 352, a timer 353, and an alarm 370. While theillustrated embodiment of the drying system 300 is automated via thecontrol system 350, neither the method nor system of the presentinvention is so limited. If desired, the functions carried out by thecontrol system 350 can be carried out manually and/or omitted in someinstances.

The helium reservoir 320, the canister 20, and the chiller 340 arefluidly connected so that a non-reactive gas, such as helium, can flowthrough the closed-loop drying system 300 without escaping into theexternal environment. More specifically, the gas supply line 325 fluidlyconnects the helium reservoir 310 to the canister 20, the gas exhaustline 326 fluidly connects the canister 20 to the chiller 340, and therecirculation line 345 fluidly connects the chiller 340 to the heliumreservoir 310, thereby forming a closed-loop gas circulation path. Allof the gas lines 325, 326, and 345 can be formed of suitable tubing orpiping. The piping and tubing can be constructed of flexible ornon-flexible conduits. The conduits can be formed of any suitablematerial, such as metals, alloys, plastics, rubber, etc. All hermeticconnections can be formed through the use of threaded connections,seals, ring clamps, and/or gaskets.

The helium gas reservoir 310 is used to store helium gas. While heliumgas is the preferred non-reactive gas for use in the present invention,any non-reactive gas can be used in conjunction with the system 300 andthe operation thereof For example, other suitable non-reactive gasesinclude, without limitation, nitrogen, carbon-dioxide, light hydrocarbongases such as methane, or any inert gas, including but not limited tonoble gases (helium, argon, neon, radon, krypton and xenon).

The supply pump 320 is operably coupled to the gas supply line 325. Whenactivated, the supply pump 320 draws helium gas from the heliumreservoir 310 and forces the helium gas into the cavity 21 of thecanister 20 via the gas supply line 325. The helium gas continues toflow through the canister 20 and into the chiller 340 via the gasexhaust line 326. The recirculation pump 360 is operably coupled to therecirculation line 345. When activated, the recirculation pump 360 drawsthe helium gas that has been de-moisturized from the chiller 340 andforces the dry helium gas back into the helium reservoir 310 for furtherrecirculation through the canister 20. While two pumps 320, 360 areillustrated as being incorporated into the drying system 300, theinvention is not so limited and any number of pumps can be used. Theexact number of pumps will be dictated on a case-by case design basis,considering such factors as flow rate requirements, pressure drops inthe system, size of the system, and/or number of components in thesystem. The direction of the helium gas flow through system 300 isindicated by the arrows on the fluid lines.

A flow rate valve 321 is operably coupled to the gas supply linedownstream of the supply pump 320. The valve 321 is used to control theflow rate of the helium gas into and through the cavity 21 of thecanister 20 and throughout the drying system 300. The valve 321 can bean adjustable flow rate valve. In other embodiments of the invention,the flow rate of the helium gas through the drying system 300 can bealternatively controlled by incorporating a mass flow rate controller.As with the pumps, any number of valves can be incorporated throughoutthe system 300 as desired. Moreover, the invention is not limited by anyspecific placement of the valve(s) or pump(s) along the closed-loop flowcircuit.

The dew-point temperature hygrometer 330 is operably coupled to the gasexhaust line 326 so that the dew-point temperature of the helium gasexiting the cavity of the canister 20 can be measured. Suitable meansfor dew point temperature measurement include direct moisture sensingdevices, such as hygrometers, and other means, such as gaschromatography or mass spectroscopy. The hygrometer 330 preferablyincludes a digital signal in some embodiments. The dew point temperaturehygrometer 330 repetitively measures the dew point temperature of thehelium gas exiting the cavity 21. There is no requirement as to thesampling rate for repetitive measurements. For example, the dew pointtemperature hygrometer 330 can measure the dew point temperature of thehelium gas multiple times per second or only once every few minutes. Insome embodiments, the time intervals between repetitive measurementswill be so small that the measurements will appear to be essentiallycontinuous in nature. The time intervals will be determined on case-bycase design basis, considering such factors as functionalityrequirements of the system and the flow rate of the helium gas.

The inlet 342 of the chiller 340 is coupled to the gas exhaust line 326while the outlet 343 is fluidly coupled to the recirculation line 345.The chiller 340 is provided to adequately de-moisturize the wet heliumgas that exits the cavity 21 of the canister 20 so that the helium gascan be re-circulated back into the helium gas reservoir 320 for furtheruse in the drying of the cavity 21. By sufficiently chilling the wettedhelium gas that exits the cavity 21 of the canister 20, the water vaporin the helium gas will condense out of the helium gas in the chiller 340and be removed via the drain 341 in liquid form. The exact temperatureto which the wetted helium gas will be chilled will depend on thedesired level of dryness. The greater the level of dryness desired, thelower the temperature. In one embodiment of the invention, it may bedesirable to chill the wetted helium gas to a temperature of 25° F. orless. Once de-moisturized in the chiller 340, the dry helium gas will bere-circulated back into the reservoir 310 for further use.

While the wetted helium gas is de-moisturized in the illustratedembodiment of the drying system 300 using a chiller 340, otherde-moisturizing apparatus and methods can be used instead of or inaddition to the chiller 340 if desired. For example, a condenser orfreezer may be used. In another embodiment, the wetted helium gas may beexposed to a suitable desiccant, such as silica gel, that will absorbthe water vapor from the wetted helium gas stream. The desiccant can bedried as necessary through heating, UV exposure, or other conventionaldrying process and subsequently reused.

In embodiments of the present invention that do not re-circulate thehelium gas, de-moisturizing the wetted helium gas will not be necessary.As such, the chiller 340 or other drying module will be omitted.

The drying system 300 further comprises an automation system 350. Theautomation system 350 comprises a CPU 351, a computer memory medium 352,a timer 353, and an alarm 370. The CPU 351 is a suitable microprocessorbased programmable logic controller, personal computer, or the like. Thecomputer memory medium 352 can be a hard drive that comprises sufficientmemory to store all of the necessary computer code, algorithms, and datanecessary for the operation and functioning of the drying system 300,such as predetermined time, predetermined dew-pint temperature, desiredchilling temperatures, flow rates, and the like. The timer 353 is astandard digitalized or internal computer timing mechanism. The alarm370 can be a siren, a light, an LED, a display module, a speaker, orother device capable of generating audio and/or visual stimulus. Whilean alarm 370 is illustrated and described, any instrumentation, device,or apparatus that inform an operator that the drying system 300 hascompleted a drying process can be used. For example, a computer screencan simply indicate that the canister is dry via text or visuals.

The CPU 351 includes various input/output ports used to provideconnections to the various components 320, 321, 330, 340, 360, 370, 352,353 of the drying system 300 that need to be controlled and/orcommunicated with. The CPU 351 is operably coupled to these componentsvia electrical wires, fiber-optic lines, co-axial cables, or other datatransmission lines. These connections are indicated by the dotted linesin FIG. 3. The CPU 351 can communicate with any and all of the variouscomponents of the drying system 300 to which it is operably connected inorder to control the drying system 300, such as: (1) activating ordeactivating the pumps 320, 360; (2) opening, closing, and/or adjustingthe flow rate valve 321; (3) activating or deactivating the chiller 340;and (3) activating or deactivating the alarm 370.

The CPU 351 (and/or the memory 352) is also programmed with the properalgorithms to receive data signals from the dew-point hygrometer 330,analyze the incoming data signals, compare the values represented by theincoming data signals to stored values and ranges, and track the time atwhich the values represented by the incoming data signals are at orbelow the stored values. The type of CPU used depends on the exact needsof the system in which it is incorporated.

Referring to FIG. 4, a flowchart of an embodiment of a method of dryinga cavity loaded with SNF according to an embodiment of the presentinvention is illustrated. The method will be described in relation tothe drying system 300 of FIG. 3 for ease of description andunderstanding. However, the method is not limited to any specificstructure or system, and can be carried out by other systems and/orapparatuses.

At step 400, the cask 10 containing the SNF loaded canister 20 ispositioned in a staging area after being removed from the coolingpool/pond. As discussed above, the cavity 21 of the canister 20 isfilled with water from the pool at this time. The bulk water is drainedfrom the cavity 21 of the canister 20 via a properly positioned drain,thereby completing step 400.

Despite the bulk water being drained from the cavity 21 of the canister20, the interior of the cavity 21 and the SNF are still moisture bearingand need further de-moisturization for long-term storage. In order tofurther dry the cavity 21 and the SNF, the drying system 300 isutilized. The canister 20 remains in the cask 10 during the dryingoperation. At step 410, the gas supply line 325 is fluidly coupled tothe inlet 28 of the canister 20 while the gas exhaust line 326 isfluidly coupled to the outlet 29 of the canister 20. As a result, aclosed-loop fluid circuit is formed in which the cavity 21 of thecanister 20 forms a portion of the fluid circuit.

Once the drying system 300 is properly hooked up to the canister 20, theanswer to decision block 420 is YES and the operator activates thedrying system 300. The drying system 300 can be activated manually byswitching on the equipment or in an automated fashion by the CPU 351.When activated in an automated fashion, an operator will activate thedrying system 300 by entering a system activation command into a userinput device (not illustrated), such as a keyboard, computer, switch,button, or the like, which is operably coupled to the CPU 351. Uponreceiving the associated system activation signal from the user inputdevice, the CPU 351 sends the appropriate activation signals to thepumps 320, 360, the chiller 340, the hygrometer 330, and the flow ratevalve 321.

Activating the supply pump 320 and the recirculation pump 360 results inthe helium gas being drawn from the helium reservoir 310 and flowedthrough the closed-loop fluid circuit (which includes the gas supplyline 325, the canister 20, the gas exhaust line 326, the chiller 340,and the recirculation line 345). The flow rate of the helium gas throughthe drying system 300 is controlled by the flow rate valve 321, which ispreferably an adjustable valve. In one embodiment to the presentinvention, the CPU 351 opens the flow rate valve so that the helium gasflows through the canister 20 at a flow rate of approximately 400 lb/hr.However, the invention is not so limited and other flow rates can beused. The exact flow rate to be used in any particular drying operationwill be determined on a case-by-case design basis, considering suchfactors as the open volume of the canister's cavity, the target drynesslevel within the canister's cavity, the initial moisture content withinthe canister's cavity, the moisture content of the helium gas maintainedwithin the reservoir, desired number of hourly volume turnovers for thecanister etc.

The chiller 340 is also activated by the CPU 351 so that the wettedhelium gas exiting the canister 20 can be de-moisturized prior to beingre-circulated back into the helium reservoir 310. In one embodiment, theCPU 351 activates the chiller 340 so that the helium gas is chilled to atemperature of 25° F. or less. However, the chiller 340 can be used tocool the helium gas to any desired temperature that suitablyde-moisturizes the helium gas. As discussed above, in some embodimentsof the invention, other de-moisturizing apparatus, such as those thatutilize a desiccant, can be used to dry the wetted helium gas instead ofthe chiller 340.

Upon being activated, the supply pump 320 draws dry helium gas from thehelium reservoir 310 and flows the dry helium gas into the wet cavity 21of the canister 20 via the inlet 28. Upon entering the cavity 21, thedry helium gas absorbs water from the SNF and internal surfaces of thecavity 21 in the form of water vapor. The moisture laden helium gas thenexits the cavity 21 via the outlet 29. As the wet helium gas exits thecavity 21, the hygrometer 330 repetitively measures its dew pointtemperature. As the hygrometer 330 measures the dew point temperature ofthe wetted helium gas, it generates data signals indicative of themeasured dew point temperature values and transmits these data signalsto the CPU 351 via the electrical connection, thereby completing step440.

Upon receiving the data signals indicative of the measured dew pointtemperature values, the CPU 351 compares the measured values to apredetermined dew point temperature value that is stored in the memorymedium 352. Thus, step 450 is completed. The predetermined dew pointtemperature is selected so as to be indicative that the inside of thecavity 21 and the SNF is sufficiently dry for long term storage. In oneembodiment, the predetermined dew point temperature is selected so as tocorrespond to a vapor pressure in the cavity 21 that is indicative of anacceptable level of dryness, such as for example 3 Torr or less. In suchembodiments, the predetermined dew point temperature can be selectedusing either experimental or simulated correlations.

Referring now to FIG. 5, an exemplary embodiment of how one selects thepredetermined dew point temperature will be described. As can be seenfrom the curve delineated in FIG. 5, the water vapor pressure of gases,such as helium, correlates to a dew point temperature. Thus, using thiscurve, the predetermined dew point temperature can be determined oncethe target vapor pressure is known. For example, if the target vaporpressure is 3 Torr, this corresponds to a dew point temperature ofapproximately 22.9° F. This position is indicated by point A on thecurve. The target vapor pressure can be mandated by a government orother regulatory organization and can vary greatly. In some embodiments,it is preferable that the predetermined dew point temperature be in therange of approximately 20-26° F., and most preferably about 22.9° F. Theinvention, however, is not limited to any specific dew point value, Theexact dew point temperature of the wetted helium gas that willcorrespond to an adequately dry state within the cavity 21 will bedetermined on a case-by-case basis, considering such factors asgovernment regulations, mandated safety factors, the type of HLW beingstored, the storage period, etc.

Referring back to FIG. 4, after the CPU 351 compares the measured dewpoint temperature to the predetermined dew point temperature, the CPU351 then determines whether the measured dew point temperature is lessthan or equal to the predetermined dew point temperature, thusperforming decision block 460. This comparison is performed for eachsignal received by the CPU 351.

If the measured dew point temperature of the wetted helium gas exitingthe canister is determined to be above the predetermined dew pointtemperature, the answer at decision block 460 is NO and the CPU 351 willcontinue to decision block 490. At decision block 490, the CPU 351determines whether the timer 353 has been activated (which is done atstep 470). If the timer 353 is activated, the answer at decision block490 is YES and the CPU 351 deactivates the timer 353 and returns to step440. If the timer 353 is not activated, the answer at decision block 490is NO and the CPU 351 returns directly to step 440. Either way, if themeasured dew point temperature of the wetted helium gas exiting thecanister is determined to be above the predetermined dew pointtemperature, the drying system 300 continues to circulate the dry heliumgas into and through the cavity 21 of the canister 20.

However, if the measured dew point temperature of the wetted helium gasexiting the canister is determined to be at or below the predetermineddew point temperature, the answer at decision block 460 is YES and theCPU 351 will continue to step 470. At step 470 the CPU 351activates/starts the timer 353. The timer 470 is programmed to run for apredetermined time. The selection and purpose of the predetermined timewill be discussed in greater detail below.

Once the timer is activated at step 470, the CPU 351 proceeds todecision block 480 to determine whether the timer 353 has expired (i.e.,whether the predetermined time has passed). If the answer at decisionblock 480 is NO, the CPU 351 returns to step 440 and the drying system300 continues to circulate helium gas through the cavity 21 of thecanister 20 and repeat the operations of steps 440-470 until thepredetermined time expires. In other words, the drying process continuesuntil the measured dew point temperature of the wetted helium gasexiting the canister falls below (or equal to) the predetermined dewpoint temperature, and remains so for the predetermined time (withoutsubsequently rising above the predetermined dew point temperature).

By requiring that the measured dew point temperature of the wettedhelium gas exiting the canister not only reach, but remain at or belowthe predetermined dew point temperature for the predetermined time, itis ensured that the cavity 21 and the SNF therein are sufficiently driedwithin firs acceptable safety factor. This, along with the means forselecting the predetermined time, will now be described with respect toFIG. 6.

Referring to FIG. 6, the affect on the dew point temperature thatcontinuing the helium gas flow through the canister 20 over time isexemplified. The data in the graph was simulated assuming a dry heliumflow rate of 400 lb/hr, a pressure of 50 psi, a moisture level of 1 mmHg within the dry helium gas, a canister volume capacity of heliumholdup of 10 lb, and an initial canister moisture level of 100 mm Hg. Ascan be seen from the graph, at time (“t”)=0.1 hours (i.e., 6 minutes),it can be estimated that the dew point temperature within the cavity 21is at about 22.9° F. (which from FIG. 5 corresponds to a vapor pressureof about 3 Torr), indicated on the graph as point B. As the flow ofhelium gas through the cavity 21 is continued over time, the dew pointtemperature will continue to decrease until an equilibrium vaporpressure is reached, which in the graphed example is at about t=0.36hours (i.e., about 22 min), indicated on the graph as point C. Ifdesired, the flow of helium gas through the cavity cart be furthercontinued, but it will not result in any further significant decrease ofthe dew point temperature within the cavity 21.

Taking points B and C as the points of reference, the predetermined timefor this example is about 16 minutes (i.e., from 6 minutes to 22minutes). However, if desired, the predetermined time can be less thanor greater than 16 minutes for the example. The exact predetermined timefor any situation will be determined on case-by-case design basis,considering such factors as open canister volume, flow rate, desireddryness within the cavity, desired or mandated safety factors, etc. Insome embodiments of the invention, the predetermined time willpreferably be in the range of 20 to 40 minutes, more preferably in therange of 25 to 35 minutes, and most preferably approximately 30 minutes.

Referring back to FIG. 4, once the predetermined time expires, and themeasured dew point temperature remains at or below the predetermined dewpoint temperature for the entire predetermined time, the CPU 351 arrivesat decision block 480 again. However, the answer is now YES and the CPU351 continues to step 510. At step 510, the CPU 351 generates shut downsignals that are transmitted to the pumps 320, 360. Upon receiving theshutdown signals, the pumps 320, 360 are deactivated and the flow ofhelium gas through the drying system is ceased. Alternatively, the CPU351 can cease the helium flow by closing the valve 321.

Once the pumps 320, 360 are deactivated, the CPU 351 generates andtransmits an activation signal to the alarm 370, thereby completing step520. Upon receiving the activation signal, the alarm 370 is activated.Depending on the type of device that is used as the alarm 370, theresponse of the alarm 370 to the activation signal can vary greatly.However, it is preferred that the alarm's 370 response be some type ofaudio and/or visual stimuli that will inform the operator that thecanister 20 is dry. For example, activation of the alarm 370 cangenerate a sound, display a visual representation on a computer screen,illuminate an LED or other light source, etc.

Upon being informed by the alarm 370 that the cavity 21 of the canister20 and the SNF is sufficiently dried, the operator disconnects thedrying system from the canister 20 and seals the canister 20 forstorage, thereby completing step 530.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in this art, the invention may be embodied in other specificforms without departing from the spirit or essential characteristicsthereof.

Specifically, in some embodiments, the drying method of the inventioncan be carried out manually. In such an embodiment, the pumps and allother equipment will be activated/controlled manually. The readings bythe hygrometer can be visually observed by the operator and the timingsequence operations can be performed manually.

What is claimed is:
 1. A method of preparing spent nuclear fuel for drystorage drying comprising: a) flowing a non-reactive gas through acavity containing spent nuclear fuel; b) repetitively measuring dewpoint temperature of the non-reactive gas exiting the cavity; c) uponthe dew point temperature of the non-reactive gas exiting the cavitybeing measured to be at or below a predetermined dew point temperature,activating a timer; d) running the timer for a predetermined period oftime to ensure a dryness level of the cavity; e) discontinuing the flowof the non-reactive gas after lapse of the timer; and f) sealing thecavity.
 2. The method according to claim 1, wherein step (a) comprisesflowing the non-reactive gas through the cavity containing the spentnuclear fuel at a predetermined flow rate.
 3. The method according toclaim 2, wherein the predetermined time is based on the predeterminedflow rate and volume of the cavity.
 4. The method according to claim 3,wherein the predetermined flow rate is chosen so that the volume of thecavity is turned over 25 to 50 times per hour.
 5. The method accordingto claim 1, wherein the predetermined dew point temperature is selectedto correspond to a desired vapor pressure within the cavity.
 6. Themethod according to claim 1, wherein the predetermined dew pointtemperature is in a range of approximately 20 to 26° F., and thepredetermined time is in a range of approximately 25 to 35 minutes. 7.The method according to claim 6, wherein the predetermined dew pointtemperature is approximately 22.9° F. and the predetermined time isapproximately 30 minutes.
 8. The method according to claim 1, whereinafter step b) drying the non-reactive gas with a chiller.
 9. The methodaccording to claim 1, wherein the non-reactive gas is nitrogen, carbondioxide, light hydrocarbon gases, or a noble gas selected from the groupconsisting of helium, argon, neon, radon, krypton, and xenon.
 10. Themethod according to claim 9, wherein the non-reactive gas is helium. 11.The method according to claim 1, wherein the predetermined dew-pointtemperature is selected to correspond to a vapor pressure of 3 Torr orless in the cavity.
 12. The method according to claim 1, furthercomprising drying the non-reactive gas that exits the cavity with adrying apparatus after step b).
 13. The method according to claim 12,further comprising re-circulating the dried non-reactive gas backthrough the cavity after drying the non-reactive gas in a closed flowloop.
 14. A method of preparing spent nuclear fuel for dry storagedrying comprising: a) flowing a non-reactive gas through a cavitycontaining spent nuclear fuel; b) repetitively measuring dew pointtemperature of the non-reactive gas exiting the cavity; c) drying thenon-reactive gas that exits the cavity with a moisture removal apparatusafter the dew point temperature is measured; d) re-circulating the driednon-reactive gas back through the cavity after drying the non-reactivegas in a closed flow loop; e) upon the dew point temperature of thenon-reactive gas exiting the cavity being measured to be at or below apredetermined dew point temperature, activating a timer; f) running thetimer for a predetermined period of time to ensure a dryness level ofthe cavity; g) discontinuing the flow of the non-reactive gas; and h)sealing the cavity.
 15. The method according to claim 14, wherein themoisture removal apparatus is a chiller.
 16. The method according toclaim 14, wherein step d) is performed using a recirculation pumpdownstream of the moisture removal apparatus in the closed flow loop.17. The method according to claim 16, further comprising a gas reservoirfluidly coupled in a serial flow arrangement in the closed flow loopdownstream of the recirculation pump, wherein the recirculation pumpforcible flows the non-reactive gas through the gas reservoir.
 18. Themethod according to claim 17, further comprising a gas supply pumpfluidly coupled in a serial flow arrangement in the close flow loopdownstream of the gas reservoir, wherein activating the gas supply pumpand the recirculation pump results in the non-reactive gas being drawnfrom the helium reservoir and flowed through the closed-loop fluid. 19.The method according to claim 17, further comprising a flow rate valvearranged in the closed flow loop between the gas reservoir and thecavity, the flow rate valve configured and operable to control a flowrate of the non-reactive gas circulating through the closed flow loop,and wherein the predetermined time is based on a predetermined flow rateand volume of the cavity.
 20. A method of preparing spent nuclear fuelfor dry storage drying comprising: a) flowing a non-reactive gas througha cavity containing spent nuclear fuel; b) repetitively measuring dewpoint temperature of the non-reactive gas exiting the cavity; c) dryingthe non-reactive gas that exits the cavity after the dew pointtemperature is measured; d) re-circulating the dried non-reactive gasback through the cavity after drying the non-reactive gas in a closedflow loop; and e) upon the dew point temperature of the non-reactive gasexiting the cavity being measured to be at or below a predetermined dewpoint temperature, running a timer for a predetermined period of time toensure a dryness level of the cavity.